Efforts to purify bovine milk bombesin have revealed that it appears to comprise two chromatographically separable types of molecules: one type cross-reacts with an antiserum that recognizes the peptide sequence -Gly-Asp-Leu-Trp- of bombesin (residues 5-8); the second type comprises the milk uterokinins (m-uks), which stimulates contractility, as does bombesin, oxytocin, or vasopressin, on uterine smooth muscle while they do not contract guinea-pig ileum. The following physical and chemical properties of the m-uks have been determined: 1) they exhibit an apparent Mr in excess of 1800 daltons and, thus, are larger than bombesin; 2) m-uks possess cationic, but no free anionic, functionality; 3) at least two populations of m-uks are separable by cation exchange chromatography; 4) copper- chelation chromatography results indicate the m-uks may contain histidine, tryptophan, free amino groups, and/or cysteine; and 5) uv-absorption spectra of the most highly purified preparations are compatible with the possibility that m-uks contain tryptophan and histidine. Efforts to achieve final purification are continuing with the objective of elucidating the structures of these compounds.